Includes all varieties of Corundum (Al₂O₃) which are not red (ruby). Sapphire and ruby are next in hardness to diamond being number 9 on the Mohs scale of hardness. They both crystalize in the trigonal system, often forming bipyramidal barrel-shaped crystals. They can originate by interaction of alumina-rich magmas with carbonate-bearing rocks (limestones) or as inclusions within alkaline magmas such as some types of basalt.
Blue is the most well-known and sought after colour of sapphire. The best blue is a strong but not too bright blue, royal blue or what is commonly known as a “cornflower blue”. Varying lighter varieties of blue are also common. Blue sapphire can also display secondary colours such as green, grey, and violet, which can adversely affect appearance and hence value. Dichroism is a feature common in all sapphire and ruby. It is a phenomenon where the gemstone exhibits different colour based on how the gem is oriented.
Luster is commonly high in sapphire and features such as colour zoning and silkiness are possible. Heavy silk can lead to a phenomenon stone called star sapphire.
Lesser quantities of silk can be eliminated through heat treatment. In this process the stone is subjected to a temperature of 1500 to 1600 degrees centigrade in the absence of oxygen (reducing environment). Under these conditions the rutile which causes the silk is resorbed. The titanium of the rutile then interacts with the iron in the sapphire to produce the sought after colour of blue sapphire. This improvement of colour is a process sanctioned by the gem industry as long as full disclosure of the improvement is made. This process only involves the changing of chemical components already contained within the stone. Colour can also be affected by addition of the colour-producing components in what is known as diffusion process. This process is a somewhat less accepted technique which must be disclosed to the purchaser.
Inclusions known as veils or feathers are common. These can be other included minerals, fluids and/or gas bubbles.
Weights for blue sapphire can exceed 10 carats, but are uncommon especially in better grades. These are generally pricey as a result, with values exceeding $4,000 US per carat.
The best variety of blue sapphire was originally found in Kashmir province at the disputed border of India and Pakistan. Up until the late 90’s, Australia was the world’s leading producer of blue and blue-green sapphire, but since the discovery of the huge alluvial deposits in Ilakaka and the hard-rock deposit of Andranondambo, Madagascar has claimed the number one position. Other blue sapphire countries include; Myanmar, Australia, Cambodia, Sri Lanka, Tanzania, Malawi, Rwanda, and the Helena, Montana area of the United States.
Other stones that can be confused with sapphire include blue zircon, tanzanite, some indicolite tourmaline, and cordierite (iolite).
Fancy sapphires include all colours apart from red (ruby) or blue.
Pink sapphire ranks next in value and popularity to blue sapphire.
It can exhibit colours approaching that of ruby or it may have overtones of violet or orange. The best pink sapphire can be described as being “bubble gum pink”, a vibrant pure bright pink.
Like blue sapphires, pink, violet, yellow and orange varieties exhibit remarkable luster as well as liveliness (sparkle). This is noticeable in comparing pink sapphire to pink tourmaline. The high specific gravity of sapphire also means that a stone of similar dimensions will weigh more than a pink tourmaline. The rarest variety of sapphire is called padhparasdsha. It is a variety of pink sapphire with an orangey overtone similar to a lotus blossom. Prices for this variety of sapphire can exceed that of ruby.
80 % of the sapphires produced from the sapphire fields of Ilakaka, Madagascar are pink, violet, orange and yellow. This has moved Madagascar into the number one source for fancy sapphires. Other major sources include Tanzania, Sri Lanka, and Myanmar. Fancy sapphires are produced in lesser quantities from numerous other African countries as well as Australia and The US.
Ruby is the most valuable variety of corundum. Its glowing red colour is likened to a glowing ember. Dichroism lends itself to producing a colour shift depending on the direction from which the gem is viewed. One often sees a shift from brick red in one direction to carmine in another. Colour is also enhanced by fluorescence produced by chromium in the crystal structure. This makes the red of ruby more intense in artificial light and even more so in sunlight.
Presence of other colour-modifying elements can lead to purplish to brownish overtones which can drastically shift value downwards.
Similarly to sapphire, ruby can have varying degrees of silk produced by exsolved rutile needles within the ruby. Large amounts of such silk can produce asterism (star ruby) while lesser amounts can be heated in the presence of oxygen to re-dissolve the rutile.
Spinel is the gemstone most easily confused with ruby. It does not, however, exhibit dichroism that ruby does. Other gems that may be confused with ruby include some pyrope garnet and red tourmaline. Garnet is generally less dense and is singly refractive, while tourmaline is much less dense and is significantly less bright in comparison.
Myanmar remains the location of ruby lore. Some of the largest, most valuable and famous stones have originated there. The Magok rubies of Myanmar (formerly Burma) often have the true vermillion red, with a slight pinkish overtone. Thailand rubies are typically not as intense red with purplish or brownish overtones. These deposits are on the border with Cambodia, and rubies are found on the Cambodia side of the border as well. India, Pakistan and Afghanistan also produce ruby, but generally of lower quality. Tanzania has been producing ruby from Longido and to some extent from the Tunduru area. Significant often good quality ruby has been found at Andilamena and the Tamatave area of Madagascar. Vietnam has produced a fine pinkish-coloured ruby in the past, but much of the production lately has been star rubies.
Ruby can exceed diamond in value, carat for carat, and stones of good quality over 3 carats are rare.
Chemically very similar to corundum, spinel is a mixed oxide of Aluminum and Magnesium. Iron, Manganese, Zinc, Aluminum and /or Chromium can substitute for Magnesium in its chemical structure. It crystalizes in the cubic system and hence is isotropic, not having the dichroism possessed by ruby and sapphire. It is 8 on the hardness scale and is formed in metamorphic processes involving limestones and schists or pegmatites. It is therefore often found in the same genetic environments as rubies and sapphires.
The red variety of spinel is easily confused with ruby. In fact the Black Prince’s ruby of the crown jewels is actually a red spinel that was originally thought to be ruby. When it is intense red, it can approach real ruby in value. Even though it is not dichroic, it can display a similar fluorescence to ruby. Blue spinel can also be confused with blue sapphire, as is the case for the other colours of spinel (purple, green, yellow, orange and pink) and their sapphire counterparts.
Countries producing spinel include; Myanmar, Madagascar, Tanzania, and Vietnam
Emerald belongs to a wide class of Beryllium Aluminum Silicates called Beryl. It is joined with Aquamarine (blue), Heliodor (yellow), Morganite (pink), Goshenite (colourless), and Bixbite (red). All beryl crystallizes in the hexagonal system, generally forming elongated hexagonal crystals with flat terminations. Hardness can vary from 7.5 to 8.0 and density from 2.65 to 2.90 grams per cubic centimeter.
Emerald is the light to dark green, forest green, to leaf green variety of beryl. Colour saturation must be high enough to not be confused with common beryl. The chromophore (colouring agent) in emerald can be Chromium or Vanadium or mixtures of the two. The presence of chromium generally gives the colour a boost by virtue of fluorescence. The luster is vitreous (glassy) and inclusions are almost always present to some degree. Although the colour of some tourmalines can approach that of emerald, the intense pleochroism of tourmaline is a distinguishing feature. Other gemstones that may be confused with emerald are Peridot and chrome diopside, but the colour of emerald is so distinctive that the confusion is much diminished.
The type location for emerald is the Muzo and Chivor Mines of Colombia where the emeralds are found within metamorphosed carbonaceous shale. Numerous other mines are found on the two limbs of a great fold in this area. Brazil also produces large quantities of emerald in the province of Minas Gerais. Fine quality emerald is also produced from the Hindu Kush Mountains of Afghanistan, The Swat Valley of Pakistan, as well as several east African countries including Zambia, Zimbabwe, and Tanzania. Smaller very nicely coloured emeralds are now being found in portions of SE and S Madagascar.
Aquamarine refers to that variety of beryl characterized by a light to intense blue or blue-green colour. In the variety known as Santa Maria, the blue approaches that of sapphire. The luster is vitreous and dichroism is common. Unlike tourmaline or topaz where the most intense colour is viewed down the long axis (C-Axis), the strongest colour in Aquamarine is viewed perpendicular to the long axis of the crystal. This has important ramifications as to how the stone is oriented for cutting, since more intense colour is more valuable.
The colour of Aquamarine is often enhanced through heating. This process is used to diminish the green overtones and render a purer, deeper blue. An unfortunate side-effect is to produce a steely grey overtone. By controlling temperature carefully, a remnant of the greenish overtone can be retained to produce a warmer effect, while intensifying the blue.
Generally, Aquamarine is formed in pegmatitic deposits along with other varieties of beryl as well as tourmaline and quartz. Large amounts are found in Brazil as well as Madagascar, and parts of the Russian Republic. Significant quantities are also produced from Zambia (rumoured to be among the best in the world), Mozambique and Afghanistan.
It may be confused with Blue Topaz, but Blue Topaz does not display the pleochroism of Aquamarine and is also denser. It can also be mimicked by light blue synthetic spinel or even blue glass. Larger deeper coloured stones are more rare and hence more valuable. Greenish overtones lead to stones of lesser value.
This pinkish variety of Beryl is named after the famous American banker and gem enthusiast, J.P. Morgan.
It can have a light pink or slightly orangey-pink hue to darker hues, approaching red. It has all of the other attributes of beryl, including a vitreous luster, and weak to moderate pleochroism.
It can be easily confused with Kunzite, Pink Topaz, or some Pink Tourmaline. It is less often confused with Pink Sapphire which has a greater luster.
As is the case with other Beryls, Morganite is found as a pegmatite accessory mineral. Main producers include Brazil and Madagascar.
Golden Beryl is the yellowish variety of Beryl with colour ranging from a weak canary to intense canary yellow or strong orange-yellow. It may possess a slight greenish overtone in which case it is known as Heliodor. Again it has all the other attributes of Beryl and is found in similar settings (pegmatites).
It may be confused with some varieties of Peridot or with Chrysoberyl, but would never be substituted with Chrysoberyl since it is a far rarer and more valuable gemstone than Golden beryl.
Namibia is the leading producer of Heliodor and Golden Beryl is commonly found in Brazil and Madagascar.
True Topaz is a yellow, golden brown, honey, sherry-coloured, pink to reddish orange variety of a hydroxylated, fluoro – aluminosilicate. It has fairly high density and a hardness of 8 on the Moh’s hardness scale. It forms either in pegmatites or in hydrothermally altered granitic rocks (pnematolytic deposits on the periphery of granitic intrusions). One can think of it as forming in the upper or outer portions of a granitic body that has cooked in its own juices.
Emperial or precious Topaz is generally referred to those varieties displaying an interplay of two colours, for instance yellow and orange, or yellow and pink, or sherry or pinkish varieties.
Blue Topaz, although found in nature in weak tones, is usually made by irradiation and / or heat treatment of colourless Topaz. It is common and not very valuable. It is sometimes used as a substitute for the more valuable Aquamarine.
Yellow varieties are commonly substituted with citrine and given the contrived trade name of “Citrine Topaz”. Pink varieties can be substituted with Kunzite or some pink tourmaline.
The best known deposits of Precious Topaz are those of the Ouro Preto area of Brazil. It is also found in Pakistan, The Russian Republic, Japan, Sri Lanka, and Myanmar.
Pyrope – Almandine Garnet:
This variety of Garnet is what is known as a solid solution series of Iron Magnesium Aluminosilicate. The variety where Magnesium is dominant over Iron is known as the Pyrope end member whereas the variety where Iron dominates is known as Almandine. The word “garnet” is derived from Pomegranate, to describe the tone of red. All garnet crystallizes in the cubic system and hence displays no pleochroism (usually). It has a relatively high density and a hardness of 7 to 7.5. The luster is brilliant and refractive index varies from 1.730 to 1.751 for Pyrope and 1.76 to 1.83 for Almandine.
Pyrope is generally of igneous origin being associated with more mafic rocks like basalts peridotites, eclogites and Kimberlites whereas Almandine is usually of metamorphic origin, being found in schists and gneisses.
Pyrope, due to its desirable intense redness has been used as a substitute for ruby.
Almandine, on the other hand, commonly displays a dark almost violet red colour which has come to be associated with the usual colour of garnet.
Pyrope-Almandine is commonly found in Czechoslovakia, South Africa, Madagascar, Mexico, Brazil, Australia, Myanmar, Sri Lanka, Zimbabwe, Tanzania, India, and the US.
This variety of Garnet refers to an odd mixture of Pyrope and Almandine which is characterized by a deep pink to pinkish-red colour.
As with other garnets it is none pleochroic, dense, singly refractive with a refractive index of 1.755 to 1.765.
It can be confused with pink spinel another none pleochroic pink gem. It should not be confused with pink tourmaline or sapphire due to their pleochroic natures.
Rhodolite is found in several African countries including, Tanzania, Kenya, Zambia, and Madagascar. It is also found in Sri Lanka and parts of the US.
Spessartine is a variety of Manganiferous Aluminum Silicate characterized by an orangey-pink to orangey-red or brownish-yellow colour. Hardness varies from 6.5 to 7.5 and refractive index from 1.79 to 1.81.
It is found in rocks which have been rocks which have been metamorphosed to a low grade (low heat and pressure).
Gem varieties of Spessartine are commonly solid solutions with Almandine. It is a rarer variety of garnet and clean stones exceeding 10 carats are extremely rare. Even stones over three carats are uncommon.
It was originally found in Spessart, West Germany, but is now found in somewhat greater quantities in Sri Lanka, Myanmar, Brazil, Madagascar, Tanzania, Nigeria and a number of other East African countries.
Grossular Garnet refers to those garnets composed of Calcium Aluminum Silicate, and is named for its most common colour which is gooseberry green (light yellowish green). Varieties can also vary to strong bluish green, pinkish yellow or even colourless.
Hessonite is a variety of Grossular exhibiting a honey yellow to yellowish brown colour. Despite its sometimes attractive appearance, it is not a particularly valuable variety of Grossular. Although it may resemble Citrine, it is still a superior gemstone. It is found in Sri Lanka, Brazil, Canada, US, Madagascar and numerous other Eastern African countries.
There is a variety of deep blue green to almost pure blue Grossular which atypically displays a colour change reminiscent of that of Alexandrite. This type is found in Kenya and in Madagascar.
Another deep green variety called Tsavorite is also found in Kenya, Tanzania, Pakistan, and Madagascar. Tsavorite is quite a rare variety of Grossular whose colour approaches the deep green of emerald. Stones exceeding 3 carats can be very pricey.
Andradite is a variety of garnet whose composition is Calcium Iron Silicate. Its density ranges from 3.81 to 3.86 and refractive index from 1.882 to 1.889. Its hardness is the same as other garnets being 6.5 to 7.5.
It occurs in metamorphosed limestones and less frequently in some intrusive and extrusive igneous rocks.
The pale green to clear green variety is demantoid. Colour varies from pale green to a fairly dark green. It is characterized by curved “horsetail” inclusions of asbestos and by an abnormally high dispersion (this is the feature that gives diamond its multi-colour flashes).
Demantoid could be confused with some green tourmaline, zircon, or Peridot, but none of those gems display the high dispersion of Demantoid.
It is one of the rarest garnets and gems exceeding 1 carat are uncommon and therefore expensive.
Demantoid is found in the Russian Republic and in Madagascar.
Zircon is a mineral whose composition is Zirconium Silicate which crystalizes in the tetragonal system. It commonly forms stubby prisms with bipyramidal terminations. Colours may vary from light brown, grey, brown, yellow, reddish, green, blue, greenish blue, or colourless.
High Zircon can reach densities of up to 4.70 g/cc and refractive indices up to 2.01. Hardness is about 6.0 to 6.5, and it is characterized by high dispersion (diamond-like flashes of colours).
It forms in intrusive igneous rocks, but can be remobilized into pegmatites derived from those igneous rocks. It also can form in metamorphic schists.
It is produced in Sri Lanka, Cambodia, Vietnam, Thailand, Norway, Sweden, Russian Republic, US, Madagascar.
Blue Zircon or Starlite is an electric blue variety of Zircon usually produced by heating brownish varieties from Cambodia, Vietnam or Thailand. The best Blue Zircon is that from Cambodia.
Tourmaline refers to a group of minerals whose composition is Sodium, Calcium, Lithium, Magnesium, Aluminum, Iron Aluminum Borosilicate. It often is found as elongated well terminated crystals with bulging triangular cross section.
Hardness is 7.0 and density varies from 3.02 to 3.20 g/cc. Refractive index is about 1.62 and pleochroism is strong.
The most common colour is black (variety schorl), whereas other colours include brown, pink, green, greenish-blue, blue and red. Crystals are often encountered that display concentric colour variations (watermelon tourmaline) or colour variations along the length of the crystal (bicoloured or poly-coloured tourmaline).
It is a common accessory mineral in pegmatite deposits as well as some pnematolytic deposits. The best known sources include Sri Lanka, Russian Republic, Afghanistan, Myanmar, Brazil, Tanzania, Namibia, & Zimbabwe.
This variety of Tourmaline is that which displays a pink to reddish colour. Colour is somewhat reminiscent of ruby but not nearly as bright. Secondary modifying colours can be violet or brown. As with most tourmalines, Rubellite often displays the strongest colour down the long axis (c axis) of the crystal.
It is found in Russian Republic, US, Myanmar, Sri Lanka, and Madagascar as well as several other Eastern African countries. In the past decade a vibrant reddish Rubellite has been produced in Nigeria.
Indicolite refers to those varieties of tourmaline which are essentially deep blue to slightly greenish blue. Typically the darkness of the c-axis colour often produces some extinction of colour near the ends of stones, particularly those cut in elongated emerald cuts or long ovals.
Greenish blue varieties without extinction problems are very attractive and can command prices comparable to those of Rubellite.
Indicolite is found in Brazil, Russian Republic, Namibia, Madagascar and US.
Verdilite (Green Tourmaline):
Green is a very common colour for tourmaline and there are a multitude of different possible shades. Olive greens and deep forest greens are more common, but light paintbox green and minty shades are also possible. The darker shades can be plagued with the same extinction problems that can occur with darker versions of Indicolite.
Green Tourmaline is found in Brazil, Mozambique, Tanzania, Namibia, Madagascar, Russian Republic and US.
Quartz encompasses that group of stones which are composed of Silicon Dioxide. It usually forms in hydrothermal environments where it often occurs as elongate hexagonal well terminated crystals. It can also occur as cryptocrystalline masses such as agate, jasper, carnelian or plasma. It also can form a chatoyant variety mixed with crocidolite known as tigereye.
Crystalline forms can occur in black colour (Smokey Quartz), clear (Rock Crystal), yellow to brownish orange (Citrine), and purple (Amethyst).
It has a density of 2.65g/cc, refractive indices of 1.544 and 1.553 (doubly refractive), and a hardness of 7.
The most valuable version of quartz is a rich purple variety of Amethyst displaying red flashes, known as Siberian colour. Although other varieties of Quartz are quite widespread, this type of Amethyst is best known to come from Zambia, Brazil, Uruguay, Madagascar, US, India, South Africa as well as many other countries.
Tanzanite (Blue Zoisite)
Tanzanite or blue Zoisite is hydrated Calcium Aluminum Silicate. It crystalizes in the orthorhombic system and forms stubby, often poorly defined crystals. In nature the colours it is encountered in are whitish to greyish, brownish or greenish and rarely bluish. When mildly heat treated these will convert to a stable violet to sapphire blue.
Tanzanite is strongly pleochroic (trichroic). It has a hardness of 6.5 but is very thermally and mechanically sensitive to shock. It is a stone that requires careful protection when incorporated into jewellery that may present exposure to strikes. It can be confused with Sapphire or with Cordierite (Iolite). The distinctive violet pleochroism of Tanzanite is, however, distinctive.
As its namesake suggests, Tanzanite comes from Tanzania from the foothills of Mount Kilimanjaro. This at the moment is the only source.
Jadeite belongs to a group of alkaline aluminosilicates known as pyroxenes. Jadeite refers to a matted intergrowth of pyroxene crystals which may be white, grey, brown, yellowish brown, reddish orange, lilac, blue-grey and a multitude of shades of green.
Its density is 3.30 to 3.36, and its hardness is 6.5 to 7. It is extremely tough by virtue of the interlocking of pyroxene crystals.
Jadeite is formed as lenticular masses or nodules in metamorphosed ultramafic rocks, but is often found as alluvial pebbles or boulders when the much more resistant jadeite nodules weather from the softer metamorphic host rock.
Jadeite Jade is characterized by a semi-opaque to almost transparent green. The optimum green is a uniform apple green very reminiscent of emerald and is known as Imperial Jade. Such stones are uncommon and can command very high prices into the thousands of dollars per carat.
The best known deposits of Jadeite Jade are those of Myanmar and Guatemala.
Similarly to Jadeite Jade, Nephrite Jade is a tough metamorphic rock composed of an intergrowth of Tremolite - Actinolite crystals (varieties of Amphibole). It is a hydrated silicate of Calcium, Magnesium and Iron.
Its hardness is 5 to 6 and density 2.90 to 3.40.
Similarly to Jadeite, it forms in the metamorphism of Magnesium and iron rich rocks.
Although less valuable than Jadeite jade, it was highly prized and used extensively in ancient Chinese art. Jadeite Jade only replaced Nephrite in this role in the Mid Eighteenth century. Colours are similar to those of Jadeite jade but it does not achieve the Imperial colour.
Nephrite is found in Russian Republic, Canada, New Zealand and to lesser extents US and Australia.
Lapis Lazuli refers to that gemstone composed of fine to medium grained aggregates of crystals of Lazulite. Lazulite is a Sodalite member of the group of minerals known as Feldspathoids. Its composition is Sodium Calcium Aluminum Sulphoxy – Silicate. Lazulite is a deep royal blue to dull violet.
Lapis Lazuli is commonly contains patches of white carbonate as well as inclusions of pyrite crystals. It may be confused with Sodalite itself.
It has a hardness of 5 to 5.5 and a density of 2.38 to 2.45.
It is formed through metamorphism or metasomatism of limestones.
Major producing countries of Lapis Lazuli include Afghanistan, Chile, Myanmar, Russian Republic, Pakistan, Canada, US, and Angola. The best material comes from Afghanistan.
Malachite / Azurite:
Malachite / Azurite is a mixed hydroxide and carbonate of Copper. It is commonly found in the weathered or altered portions of Copper deposits.
The variety Malachite is generally banded green while Azurite is a deep royal blue.
It is a soft material with a hardness of 4. It is also quite dense (3.8 g/cc). It is more commonly utilized in carvings then for jewellery since it is not very resistant to scratching. This lends it more towards pendants and earrings, rather than rings or bracelets.
It is found in Zambia, Zaire, Zimbabwe, Namibia, Chile, US, Russian Republic and Australia.
Turquoise is a hydrated Alumino-Phosphate of Copper. Although it crystalizes in the trigonal system, it is commonly found as lobate aggregates of microscopic crystals. Colour varies from very light to deep sky blue, robin’s egg blue or greenish blue to chartreuse. Density varies from 2.65 to 2.90 g/cc. It is often fairly porous and hardness can vary from chalk-like to as high as 6 on the Mohs scale.
It is formed in supergene environments in copper deposits where late stage fluids react with primary copper minerals. It can be a pure homogeneous blue or it can contain black or brownish veinlets reminiscent of spider webs.
In ancient times the most sought after turquoise came from Persia (Iran). Turquoise was also produced from Tibet, but nowadays the largest quantities are produced from New Mexico, Nevada, Arizona and California in the US.
It is a stone which has been notoriously imitated by plastics, ceramics, dyed Howalite or limestone. Lower grade turquoise is also stabilized by impregnation with resins, waxes or plastics.
Opal refers to a variety of Hydrated Silica (SiO₂·nH₂O).
It is a non-crystalline substance with body colour varying greatly.
It forms in low temperature hydrothermal alteration in siliceous volcanic rocks or in very low temperature precipitation of hydrated silica by connate waters in sedimentary rocks (Australia). It occurs as fracture fillings, cavity infillings, or replacement of fossils or as pseudomorphs after other minerals.
Common Opal has a glassy luster and breaks with a classical conchoidal fracture. It can be banded or homogeneous, opaque to transparent, and exhibits a wide range of colours.
Precious Opal is a phenomenon stone, where flashes of colour can be seen floating across the stone as it is moved in a light source. It is produced by the nature of the microscopic structure of the hydrated silica. This material actually occurs as closely packed spheres of silica. The sizes of the individual spheres determine the colour of the flashes.
Originally, Precious Opal was acquired from deposits in Hungary. White-based Precious Opal has been produced from large deposits at Coober Pedy in central Australia. Dark based (Black Opal) is produced from several deposits near Lightning Ridge NSW, Australia, as well as Andamooka, and Mintabie, Australia. Australia also produces a variety of Precious Opal known as Matrix or Boulder Opal. Precious Opal is also found in Brazil, Ethiopia and in small amounts the US.
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|Gemstones by Episode|
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ON COLOURED GEMSTONE GRADING
As is the case for diamonds, coloured gemstones are also graded and evaluated on the basis of the 4 “C’s”. These are colour, clarity, cut, and carat weight.
The colour of a gemstone is based on three components;
The HUE, which is the base colour of the gemstone. In the case of blue sapphire, the hue is blue. The hue can also have a secondary or modifier colour.
The TONE, which represents the strength of the colour. These values range from colourless through light tones to dark and very dark tones. Optimum tones in most coloured gemstones are around 80%. When the tone exceeds this degree the stone starts to appear to dark to see into the stone and value drops rapidly.
The SATURATION or CHROMA, which essentially is a measure of the grayish or brownish component, of the colour. The steeliness which is referred to in some blue sapphires is an example of a lower saturation, whereas the deep cornflower sapphires exhibit saturations which are strong to vivid.
For diamonds there is one scale of classification for clarity. This is not the case for coloured gemstones. Different coloured gemstones have different amounts of allowable inclusions or flaws. In fact coloured gemstones are graded by three main types of clarity classification. TYPE I gemstones are those that follow the strictest criteria for amounts of inclusions. Aquamarine is one such example, where an eye clean stone is not uncommon. TYPE II gemstones such as Ruby follow criteria which are more forgiving regarding amount of inclusions, and TYPE III gemstones like Emerald which follow an even more forgiving set of criteria.
How well a gemstone is cut significantly affects its face-up appeal or desirability and hence affects its value. Adherence to proper angles, proportions and flatness of facets as well as finish greatly affects the way light is returned by the gemstone to the viewer. The difference between a poorly cut gemstone and a well cut one is readily visible to even to the untrained eye. “Bellied” gemstones commonly cut in overseas countries not only are visually dull, but also have unnecessary weight which does not present any other attribute, other than to increase the carat weight of the gem, and hence its cost. These gemstones are not only dull but are cumbersome to mount.
Like buying any other commodity such as butter, two pounds of butter costs twice as much as one pound. This is roughly the same for gemstones. There is however an exception based on rarity. It is far less common to find a 5 carat Ruby than a 1 carat one. The value per carat for a 5 Carat ruby of the same grade is not five times that of a 1 carat Ruby, but rather thirty to thirty-five times. So much as is the case for diamonds; colored gemstones follow a tiered valuation based on carat weight.
A DISCUSSION ON GEMSTONE TREATMENTS
Different means of artificially improving the appearance and hence the desirability of a gemstone have been utilized since gemstones have been used for personal adornment, and the list grows daily. In fact, significant resources of the watchdogs of the industry, such as GIA, are devoted to finding the means to detect new gemstone treatment procedures. Over the past several decades treatments such as fracture filling and diffusion treatment, and irradiation have caused havoc in the industry mainly due to non-disclosure. These and many other treatments have come to be accepted in the industry, providing full disclosure is made to the buyer as to whether such treatments were or were not utilized in the finishing of the gemstone. This levels the playing field in that a natural cornflower blue sapphire, for instance, is far rarer than one that is produced either by heating or by diffusion treating a common piece of “geuda” sapphire. The natural sapphire is hence more valuable. A premium will exist for the value of a natural gemstone.